How can I block in colors quickly with grease pencil tool?
A grease pencil stroke has two colours, one for stroke and one for fill, the default setup has the fill opacity set to 0.0 so it is transparent. A palette is used to define the two colours to use. The current palette selection - the one highlighted - is used when you draw a stroke. Changing a palette setting will change all strokes using that palette.

By creating multiple palette entries you can quickly select a different option before each stroke.



The current grease pencil system uses layers and palettes. Each stroke can use a different palette choice and you can have multiple strokes on each layer.

The version used in the video (2.77) was made before the palettes were added, that is why there are two colours for each layer and only the one colour setting can be used per layer. The process is still the same, just that now you select the palette instead of the layer to choose a different colour setting.